1. Field of the Invention
The present invention relates to a suspension apparatus for vehicles capable of changing the characteristics of suspension by controlling supply and release amounts of hydraulic flow to hydraulic cylinders.
2. Description of the Related Art
A conventional suspension apparatus, an active control suspension apparatus (ACS apparatus) is well known as disclosed in Japanese Kokai No. Hei 3-182826. This ACS apparatus is provided with a hydraulic cylinder between a vehicle body and each wheel, and changes the characteristic of the suspension in accordance with a driving state by independently controlling the fluid flow amount to each hydraulic cylinder by a flow control valve.
In the conventional technique, as shown in FIG. 10, a vehicle height signal and vertical acceleration signal are collected on each wheel, and a bouncing component, pitching component, and rolling component of vehicle movement are extracted from those signals. Flow control signals (Q.sub.1, Q.sub.2, Q.sub.3) are calculated for hydraulic cylinders to suppress a movement of the vehicle body on each of bouncing mode, pitching mode, and rolling mode. The signal Q.sub.1 is a vehicle-height control signal, signal Q.sub.2 is a height displacement suppression signal, and signal Q.sub.3 is a vertical displacement suppression signal. In the conventional suspension apparatus, a pressure signal of the cylinder and a lateral acceleration signal as well as the above vehicle-height signal and vertical acceleration signal are inputted as parameters for active control of the suspension, and flow control signals Q.sub.4 and Q.sub.5 are outputted for correction of the rolling suppression control (warp/twist suppression control). Note that an acceleration is referred to as "G" hereinafter.
In the above-described conventional technique, a lateral G signal is used other than a height signal and vertical G signal in order to determine if a transitional rolling movement of the vehicle body is caused by vibration from the wheels or from the vehicle body along with turn. The rolling of the body may be caused when a part of the body is vertically vibrated or when the vehicle is turning. However, the lateral G signal is caused only when the vehicle is turning. Accordingly, when the lateral G signal has occurred, which means that the vehicle is in a turn, the suspension control for rolling suppression based on the vertical G signal is performed with consideration of the lateral G signal. On the other hand, when the lateral G signal does not occur, the suspension control based on the vertical G signal is applied stronger. Thus, the rolling suppression control can be accurately executed regardless of whether a turning operation is in process or not.
As shown in FIG. 10, the feature of the conventional technique is in that the control to generate a signal Q.sub.4 based on pressures in the four hydraulic cylinders is independently performed with the control to input a lateral G signal and generate a signal Q.sub.5. Accordingly, when a pressure difference occurs between a left cylinder and a right cylinder, the signal Q.sub.4 is generated in accordance with the difference, while when a lateral acceleration G signal occurs, the signal Q.sub.5 is generated in accordance with the acceleration.
Consider the case where the vehicle approaches and turns a corner as shown in FIG. 11. When the vehicle is located at the position A, the movement of the vehicle is a straight drive, therefore, neither signal Q.sub.4 nor Q.sub.5 is generated. When the vehicle is at the position B, the vehicle body is rolled since it has approached the corner, and the signals Q.sub.4 and Q.sub.5 are both generated. Furthermore, when the vehicle is at the position C, it is in a constant turning state. In this state, since the amount of the lateral G is constant, the signals Q.sub.4 and Q.sub.5 should be constant. However, when a pressure difference is generated between the right cylinder and left cylinder, the pressure difference generates the signal Q.sub.4, resulting in a change of the vehicle movement. Furthermore, it generates the lateral G, and the signal Q.sub.5 is changed, resulting in the change of the vehicle movement. It further causes a pressure difference. Accordingly, in the case where the vehicle turns, even when the turn has started and the vehicle is in a constant turning state, a rolling direction inversion of the vehicle body is repeated and the movement is unstable. That is, as shown in FIG. 12, the rolling control at the position C is delayed.
As shown in FIG. 10, since the control based on the cylinder pressure and control based on the lateral G signal are independently performed, if one of these tries to perform the most suitable control at some point, that control may not be preferable to the other (that is, a continuity of the rolling is insufficient). The problem arises not only when the vehicle is in turn, but also when a rolling state is generated in the ACS apparatus.